Energy partitioning in the reaction 2H2+ O2→ 2H2O on Pd(111)

Abstract

(2 + 1)-Photon resonance-enhanced multiphoton ionization (REMPI) has been used to detect desorbed H₂O molecules in a state-specific manner. We have studied the reaction on Pd(111) at temperatures between 500 and 700 K in the regime of low oxygen coverages ( <0.05 ML) resulting from adsorption of ambient O₂. D₂ is dosed using a supersonic molecular beam with pulse fluxes resulting in stoichiometric coverages of D and O atoms. We find that D₂O molecules are desorbed with a translational energy of ca. 300 K with weak, if any, dependence on the surface temperature. The angular distribution is nearly cosine in form. The spectroscopic data indicate that the D₂O is desorbed with an average energy in the rotational degree of freedom close to the surface temperature. Experiments probing vibrationally excited states yielded negative results setting an upper limit for their population. These data show that most of the excess energy of the final OD + D reaction step of ca. 100 kJ mol^–1 is not channelled into any of the degrees of freedom of the desorbed molecules. The data are interpreted such that the product water molecules reside on the surface for a time sufficient to dissipate most of the energy.